Optical-output power degradation of AlGaN-based deep-UV light emitting diodes by plasma treatment

Abstract

Optical power degradation in AlGaN-based deep-UV LEDs treated with low pressure O2-plasma treatment is reported. This process was performed prior to the metallization of p-type contacts, with and without transparent layer using disk-LEDs geometry. Following the rapid thermal annealing (RTA) of Ti/Al/Ti/Au, (n-contact) and Ni/Au, (p-contact), their current-voltage characteristics were analyzed. A significant increase in the p-contacts resistivity of the samples processed without a transparent layer was observed after treating with oxygen plasma for 15 s (at 0.25 mbar, and 30 W). This can be attributed to the generation of point-like defects and the donor-like nitrogen vacancies in the bare GaN-p-contact pads. A drastic change in the thermal stability of p-contact led to a significant loss in the optical output power in these devices. Furthermore, a noticeable decrease in the p-contact stability and their adhesion properties were also observed. This was mainly due to the Fermi level movement toward the conduction-band in p-GaN. Results on the comparative study of the L-I characteristics of the processed LED's, for their p-contacts treated with- and without-plasma are also discussed. Present study emphasizes the need to address the plasma damage related issues, as it can significantly degrade the optical output efficiencies of high-power flip chip AlGaN-based deep-UV emitters.